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Williams Power Supply +5V Adjustment

Fri, 04/06/2012 - 13:46 — mspaeth

This modification is really old, and I posted the info to RGVAC back in the mid 90's, but since others have taken the info and passed it off as their own work and left out the explanation, I figured it was worth of a repost. The standard Williams linear power supply (used in games such as Defender, Stargate, Joust, Robotron, Bubbles, Splat, Blaster, Joust II, Mystic Marathon, Inferno, etc...) has high current fixed +5V and +12V regulators onboard. Due to burned connectors and cracked solder joints, the +5V from the power supply often ends up being far lower at the PCBs where it really matters. (Gee, wouldn't a sense line like the ones on Atari linear supplies be nice? -- Oh wait, morons from the internet tell you to disable that!)

Original Circuit Operation

The stock supply generates a 5.1V reference (pin 5) from the 7.3V bandgap output of the LM723 (pin 6) using 4 fixed resistors (R10, R24-26 on later supplies), and the LM723 drives the base of the 2N6057 NPN darlington transistor which provides current to the PCB through current sense resistor R3. The voltage the boards sees is then low-pass filtered through R23 and C9 back to the inverting input of the LM723. The negative feedback loop tries to make the voltage at the inverting terminal of the amplifier (pin 5) equal the 5,1V reference voltage at the non-inverting terminal (pin 5) by servoing its output (pin 10) which drives the NPN. (We can ignore the current sense (pin 3) and current limiting (pin 2) inputs of the LM723, since they don't do anything in normal operation anyway). The overall regulator circuit from the manual looks like:

Reference Generation

As stated before the 5.1V reference is generated by a voltage divider between R24=27k in parallel with R26=2.15k (for a parallel sum of 1.991k) and R10=51k in parallel with R26=5.11k (for a parallel sum of (4.645k). This divider makes the reference voltage 7.3V*4.645k/(4.645k+1.991k)=5.109V. As stated in the manual, if the +5V output is too low, R10 may be removed to make the reference 7.3V*5.11k/(5.11k+1.991k)=5.263V, and if the +5V output is too high, R24 may removed to make the reference 7.3V*4.645k/(4.645k+2.15k)=4.99V.

The Recommended Mod

In practice, such coarse adjustment is cumbersome, so it'd be much nicer to just put a potentiometer on the board so the output voltage can be manually adjusted in fine increments. To do this:

Remove resistor R26.

Take a 10k pot (preferable one with wire leads rather than studs) and solder the wiper leg to either of the other legs

Solder the pot back to the board where R26 used to be

With the pot in place, the pull down resistance can be dialed between 0 ohms in parallel with 51k (for a parallel sum of 0 ohms) and 10k in parallel with 51k (for a parallel sum of 8.361k). thus the +5 output can be dialed all the way down to ground at one extreme of the pot, or all the way up to 7.3V*8.361k/(8.361k+1.991k)=5.896V at the other extreme. While ~5.9V shouldn't hurt anything on the board, it's good practice to start with the pot somewhere in the middle, and measure the voltage at the board while you tweak the pot until you get the desired voltage, then hot glue the pot in place so it won't drift.

Alternatives

Since others have asked in the past, it's worth explaining other ways to do the mod:

If R10 is also removed and the modified put soldered in place of R26, the voltage adjustment is a little more linear, but the maximum voltage rises to ~6.1V. This still shouldn't damage anything on the board, but there's no reason for the extra range, and that's just one more part to remove.

If we instead remove R24 and R25 and use the 10k pot as a pullup, the regulator is then adjustable between 2.43V and the full 7.3V, so we can still get the desired voltage, but a misadjusted pot can send a potentially damaging voltage to the board.

if we remove all four the of the divider resistors, and just wire the pot with the wiper to in 5 of the LM723 and the other legs to pins 6 and 7, the reference can be linearly adjusted from ground all the way up to 7.3V. This mod is simple and direct and doesn't require a specific pot resistance, so it can be useful if you don't have a 10k on hand, but it suffers from the same high(er) voltage problems as the pullup mod